1. Field of the Invention
This invention relates to a method of manufacturing a Bi (bismuth)-layered superlattice material using trimethylbismuth. This invention specifically relates to a method of forming with good reproducibility a Bi-layered superlattice material of a desired composition using trimethylbismuth.
2. Statement of the Problem
Recently, techniques for forming capacitance elements comprising superlattice materials having spontaneous polarization on a semiconductor integrated circuit have been developed in order to realize nonvolatile RAM (random access memory) which enables low operating voltage, high-speed writing and reading compared to conventional RAMs. A group of materials called Bi-layered superlattice materials are developed to be used for such a capacitance insulating film. This group of Bi-layered superlattice materials comprise complex oxides of metals, such as strontium, calcium, barium, bismuth, cadmium, lead, titanium, tantalum, hafnium, tungsten, niobium, zirconium, scandium, yttrium, lanthanum, antimony, chromium and thallium that spontaneously form layered superlattices, i.e. crystalline lattices that include alternating layers of distinctly different sublattices. Generally, each layered superlattice material will include two or more of the above metals; for example, strontium, bismuth and tantalum form the layered superlattice material strontium bismuth tantalate. These layered superlattice materials require a superlattice material generator element which is usually bismuth and is typically found in a precursor solution as bismuth oxide. A problem with many of the superlattice material generator elements is that a high vapor pressure at room temperature is required to reliably manufacture these layered superlattice materials. Another problem with present superlattice material generator elements is that they aren""t thermally stable. Many bismuth generator compounds are easily decomposed when subjected to the operating conditions of the chemical vapor deposition (CVD) method. An additional problem with the processes for fabricating the layered superlattice materials, is that the prior art processes require a temperature of 800xc2x0 C. or more sometime in the fabrication process. Generally, conventional integrated circuit components, such as CMOS components and wiring layers begin to deteriorate at temperatures higher than about 650xc2x0 C., and the higher the temperature, the more rapid the deterioration. Thus, the high fabrication temperatures of the layered superlattice materials have prevented commercially viable high-yield processes from being developed.
The above and other problems are solved and an advance in the art is made by a chemical vapor deposition method for making layered superlattice materials using trimethylbismuth in accordance with this invention. A first advantage of this invention is that the method provides, prior to chemical vapor deposition, a liquid precursor with a high vapor pressure even at room temperature. A second advantage of this invention is that the method provides, during chemical vapor deposition, a precursor vapor that is effectively vaporized. A third advantage of this invention is that the precursor vapor is stable at high temperature. Trimethylbismuth has a high vapor pressure at room temperature and it""s decomposition temperature is higher than many of the present bismuth generator elements compounds. A fourth advantage of this invention is that the method provides chemical vapor deposition of Bi-layered perovskite materials with good reproducibility. A fifth advantage of this invention is that the substrate is annealed at a temperature below 650xc2x0 C.
The invention provides a method of making a thin film layered superlattice material on an integrated circuit substrate using trimethylbismuth, the method comprising the steps of: dissolving the trimethylbismuth and a metal compound in a solvent to produce a final precursor liquid; forming a mist of the final precursor liquid with a carrier gas to produce a precursor mist; vaporizing the precursor mist to produce a precursor vapor; providing the integrated circuit substrate; applying the precursor vapor onto the substrate to produce the thin film superlattice material without any heating step that includes a temperature of 650xc2x0 C. or higher; and completing the fabrication of the integrated circuit to include at least a portion of the thin film layered superlattice material in a component of the integrated circuit. Preferably, the step of dissolving the trimethylbismuth comprises mixing the trimethylbismuth with an organic solvent. Preferably, the metal compound comprises a metal polyalkoxide compound containing at least two metals selected from the group consisting of strontium, calcium, barium, cadmium, lead, tantalum, hafnium, tungsten, niobium, zirconium, bismuth, scandium, yttrium, lanthanum, antimony, chromium, molybdenum, vanadium, ruthenium and thallium. Alternatively, the metal compound comprises a diketonate compound containing at least two metals selected from the group consisting of strontium, calcium, barium, cadmium, lead, tantalum, hafnium, tungsten, niobium, zirconium, scandium, yttrium, lanthanum, antimony, chromium, molybdenum, vanadium, ruthenium and thallium. Preferably, the metal compound comprises an oxide compound containing at least two metals selected from the group consisting of strontium, calcium, barium, cadmium, lead, tantalum, hafnium, tungsten, niobium, zirconium, scandium, yttrium, lanthanum, antimony, chromium, molybdenum, vanadium, ruthenium and thallium. Preferably, the amount of trimethylbismuth is less than 15 mol %. More preferably, the amount of trimethylbismuth is less than 10 mol %. Most preferably, the amount of trimethylbismuth is 8 mol %. Preferably, the organic solvent comprises at least one compound selected from the group consisting of tetrahydrofuran, methyl ethyl ketone, isopropanol, methanol, xylene, n-butyl acetate, octane, 2-methoxyethanol, toluene, diethylethane, 1,4-dioxane and hexane. Preferably, the step of forming a mist of the precursor liquid comprises flowing the carrier gas across an open throat of a tube containing the precursor liquid, with the flow of the gas being substantially parallel to the open throat. Preferably, the method further includes a flowing oxygen gas into the deposition chamber reactor prior to the step of reacting. Preferably, the method further comprises at least one step of treating the thin film at temperatures below 650xc2x0 C. to crystallize or partially crystallize the superlattice material in a phase including more grains with a high polarizability orientation than prior to the at least one step of treating, whereby the at least one step of treating is RTP, oxygen furnace annealing prior to formation of an electrode or contact to the layered superlattice material thin film, or an anneal after an electrode or other contact to the superlattice material thin film is formed. Preferably, the step of mixing the precursor mist comprises mixing the precursor mist with an inert carrier gas. Preferably, the carrier gas comprises at least one compound selected from the group consisting of nitrogen and argon. Preferably, the step of vaporizing comprises heating the precursor aerosol prior to the deposition step. Preferably, the step of heating further comprises heating the precursor aerosol to a temperature of from 50xc2x0 C. to 250xc2x0 C. Preferably, the step of heating further comprises heating the precursor aerosol to a temperature of from 100xc2x0 C. to 200xc2x0 C. Preferably, the step of providing the substrate further comprises placing the substrate inside a deposition chamber. In one preferred embodiment, the step of applying the precursor vapor comprises chemical vapor deposition. Preferably, the step of applying the precursor vapor further comprises deposition on a heated substrate. Alternatively, the step of applying the precursor vapor further comprises deposition on an ambient temperature substrate. Preferably, the thin film layered superlattice material is 1 nm to 100 nm thick.
The invention also provides a method of making a thin film layered superlattice material on an integrated circuit substrate using trimethylbismuth, the method comprising the steps of: dissolving the trimethylbismuth and a metal compound in a solvent to produce a precursor liquid, with the relative concentration of the trimethylbismuth to the solvent being 15 mol % or less; forming a mist of the precursor liquid with a carrier gas to produce a precursor mist; vaporizing the precursor mist to produce a precursor vapor; providing the integrated circuit substrate; applying the precursor vapor onto the substrate to produce the thin film superlattice material without any heating step that includes a temperature of 650xc2x0 C. or higher; and completing the fabrication of the integrated circuit to include at least a portion of the thin film layered superlattice material in a component of the integrated circuit.
In another aspect the invention provides a precursor solution for forming a layered superlattice material, the precursor comprising trimethylbismuth and a metal compound in a solvent, with the relative concentration of the trimethylbismuth to the solvent being 15 mol % or less. Preferably, the metal compound comprises a metal polyalkoxide compound containing at least two metals selected from the group consisting of strontium, calcium, barium, cadmium, lead, tantalum, hafnium, tungsten, niobium, zirconium, bismuth, scandium, yttrium, lanthanum, antimony, chromium, molybdenum, vanadium, ruthenium and thallium. Preferably, the metal compound comprises a diketonate compound containing at least two metals selected from the group consisting of strontium, calcium, barium, cadmium, lead, tantalum, hafnium, tungsten, niobium, zirconium, scandium, yttrium, lanthanum, antimony, chromium, molybdenum, vanadium, ruthenium and thallium. Preferably, the metal compound comprises an oxide compound containing at least two metals selected from the group consisting of strontium, calcium, barium, cadmium, lead, tantalum, hafnium, tungsten, niobium, zirconium, scandium, yttrium, lanthanum, antimony, chromium, molybdenum, vanadium, ruthenium and thallium. Preferably, the amount of trimethylbismuth is 10 mol % or less. More preferably, the amount of trimethylbismuth is 8 mol %. Preferably, the solvent comprises at least one compound selected from the group consisting of tetrahydrofuran, methyl ethyl ketone, isopropanol, methanol, xylene, n-butyl acetate, octane, 2-methoxyethanol, toluene, diethylethane, 1,4-dioxane and hexane.
The present invention not only provides a more easily controlled process for making the layered superlattice materials, but also provides a method that is much more compatible with conventional integrated circuit processes. Other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.